4 Answers
4

Technically, the sentence is not grammatical. The speaker is using the subjunctive mood to express something hypothetical and/or counter-factual (in reality, they are not in a physical relationship). A more grammatically precise rendering would use the subjunctive past imperfect in conjunction with the conditional:

If we were in a physical relationship, you would have just lost sex tonight.

In spoken, colloquial language, the sentence rendered as it was in the TV programme is nonetheless perfectly intelligible, although I'm sure there are lots of self-declared English language authorities who would issue red-faced, frothy-mouthed diktats to the contrary. I will note, however, that as an Englishman this rendering sounds very American to my ear.

"...self-declared English language authorities who would issue red-faced, frothy-mouthed diktats..." -- There are, of course, none of those to be found in this forum... are there? (Nice turn of phrase, BTW. :-) As you imply, the nub of this issue is the register and context of the utterance. I watched that episode: Sheldon was giving his platonic girlfriend Amy a mild telling-off. In that context, I find nothing to criticize in what he said (other than the fact that he actually said it. ;-)
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Erik KowalMay 7 '14 at 10:04

@ErikKowal ha! So it was arch-pedant Sheldon who was mis-matching his subjunctive and indicative moods :)
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568mlMay 7 '14 at 13:10

An untypical lapse on Sheldon's part. The dismay his friends must have felt when he made it can only be imagined. :-)
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Erik KowalMay 7 '14 at 20:21

The first part is expressing a conditional which is not real. We are not in a physical relationship, but if we were, then what follows would be true.

The second part means that the person who is addressed just did something that caused him not to have sex with the speaker. However, he was not going to have sex with the speaker, as the speaker and the addressee are not in a physical relationship.

For the us of this kind of conditional, I believe there is no better reference than the famous song If I were a carpenter:

If I were a carpenter
And you were a lady,
Would you marry me anyway?
Would you have my baby?

I agree. It's a condition and a cause in a present tense.
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TuckerMay 7 '14 at 10:17

I'm afraid it's not fine. After a past-tense "if" (representing a hypothetical/unreal condition), you need a conditional in the consequence. Even the song has it - "would," not "will."
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AngewMay 7 '14 at 10:40

The song illustrates the first clause. About needing a conditional in the second clause, 568ml has worded a nice opinion about that, which I gladly will second. As to whether the sentence as a whole would withstand the intense scrutiny of trigger-happy prescriptionists, I agree, it would not. I guess the question is whether grammar is what the rules say, or grammar is how people use the language in order to be understood without any problems. And for those who prefer the second definition, the sentence is fine :)
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oerkelensMay 7 '14 at 11:19

There is a mis-match that is strictly incorrect, but which has rhetorical value.

The first clause is in the subjunctive mode, and so the second part should also be, or at the very least the switch should be less abrupt than it is.

However, consider that the indicative mode of the second clause is much more immediate. "you just lost sex" is much more of a "burn" than "you would have just lost sex".

There's also a further element of humour in this (it is a comedy show after all), in that the speaker has switched from talking about a hypothetical to talking as if it was the case. As well as being part of the increase in immediacy I just mentioned, this also addresses the nature of the relationship in the show, which is (as stated) not sexually intimate, but where it is known that the character addressed wants it to be.

As such, the "incorrect" grammar could be reasonably considered artistic license; not a good idea normally, but used here for effect.